Cas hung back awkwardly at the door to Dean's room, leaning against the doorframe. Sam was talking to Dean as though the two were reminiscing over a round of beers. The forced cheeriness in Sam's voice was relentless, unwavering despite the futility of conversing with a dying comatose patient.

Cas sighed. He was being harsh, he knew. Unable to find anything remotely helpful in the lore or online, Sam was doing the only thing he could think of to help his brother: reminding him of the good old days.

Right now he was just coming to the end of a lengthy account of Sam's graduation from high school. He had resigned himself to spending the day more or less alone. He'd even made plans to celebrate with his friends for a while before they were all brought out to dinner by their proud families. Dean and their father had been on a wendigo hunt one state over that had dragged on two days longer than they had originally thought. Sam described his odd sense of loneliness, graduating with a then-secret full ride to Stanford and a near-perfect GPA among a sea of smiling faces. This was an accomplishment he was proud of, yet he couldn't share it with his father or brother. As he and the other graduates waited in line to receive their diplomas, Sam had stared dejectedly at the hem of the gown in front of him, not wanting to see the crowd of strangers. This amplified his surprise when he gave an awkward bow to the onlookers after shaking his principal's hand and heard a familiar whoop. Looking up, he had seen a dirty and bruised Dean standing by the fire exit, clapping enthusiastically along with everyone else and beaming at Sam. After the ceremony, Dean, having excused himself to the men's room to scrub some of the more obvious dirt from his face, had taken Sam out to dinner in an expensive local restaurant. When Sam had protested over the price of the meal – having tried and failed to get away with just ordering a salad – Dean had confessed that he'd been saving his hustling money for weeks to treat Sam on his "big nerd day".

The story made Cas smile. He admired the innate humanness of it. It was odd to him how much a meal with a brother could mean to someone. When Sam finished telling the tale, he smiled down at his brother. Cas could almost feel the hope radiating off him. He didn't need to look into the hazel eyes to know what Sam was thinking. Maybe this story will snap him out of it. Maybe this one.

When Dean remained as motionless as had been for the past two weeks, Sam's shoulders slumped, almost imperceptibly. Cas cleared his throat.

"Any change?"

Sam shook his head. He leant back in his chair with a sigh. "Nothing."

Cas stepped over the threshold and sat in his chair on Dean's other side. He was two-thirds a shade paler than he had been that morning. Sam had shaved his 'demon beard' some time ago, and his usual stubble looked like flecks of dirt against the grey-tinged skin that was almost the same colour as the cannula. His cheekbones now cast noticeable shadows.

"Maybe I should try the Dream Root again ..." Sam muttered, half to himself, Cas thought.

"I don't think that would be wise."

"You didn't think it was wise last time," Sam pointed out.

"True. But he's even weaker now. It took a toll on him, Sam. I don't think he has the strength for it anymore."

Air huffed angrily out through the hunter's nostrils.

"Then what are we meant to do, Cas?" His voice was as tight as his jaw. "He's dying, Metatron's creating an angel army, there's a Knight of Hell still out there, not to mention god knows how many demons who're –" He stopped himself. Cas listened to his deep breathing for a moment, looking miserably at Dean. "Everything is wrong and there's nothing we can do."

The question Cas had been wrestling with for the past three days finally found its answer in Sam Winchester's unbearably sorrowful and defeated face. Taking a deep breath, Cas committed himself. "There is one thing that might help."

Sam looked at him sharply. "What?"

Finding his interlocked fingers to be an easier thing to look at than the hunter's laser-like gaze, Cas elaborated. "I had an idea. Dean isn't a demon anymore; we know that. We also know that the Mark is, at least for now, contained, and that containment appears to be holding. We think. But." Cas looked up at Sam. "It is possible that there is some lingering ... infection in Dean's body."

"What d'you mean? Like some of the demon taint's still there?"

"Something like that."

"But I thought we burnt that away with Grace and blood?"

"We did. It's not likely there's more than a few molecules – if any – left affecting Dean, but even if there isn't, the cure took a lot out of him."

Sam huffed. "Yeah. I noticed."

"Well, what if his body isn't producing new, pure blood? What if it's the same blood that was tainted still flowing through him?"

Sam glanced from Cas to Dean. His expression turned suspicious. "Cas, you once diagnosed a dead guy with a urinary infection by sniffing him. What are you trying to say?"

Cas smiled. Then it faded. "Part of the reason Dean needs the IV and oxygen is because his organs are failing. His liver, kidneys, even his lungs are slowly shutting down. He is producing new blood cells, but for some reason I can't explain for certain, they're not helping his condition when they should be. It's like ..." Cas cast his eyes around the small room, trying to think of simple enough words to explain what he intuitively knew to Sam. It was a complex concept. "Dean's nuclei aren't absorbing the energy they needed to function, and given the unusual circumstances of his illness, I think there's more to it than a random failing."

"So," Sam drawled, clearly wondering where this was going, "how do we fix it?"

"I think ... a blood transfusion."

Sam's eyebrows flicked upwards.

"What?" Cas asked, straightening up.

"Nothing. I just thought it would be something a bit more complicated than that, that's all," he said haltingly.

"Ah. Well," Cas began.

"Lemme guess," Sam groaned, "it's about to get complicated?"

Cas chuckled humourlessly. "Yes. It's hard to explain without using Enochian, but I think that if we give Dean a transfusion of pure, innocent blood, it would help him fight whatever battle is going on inside his head."

Sam studied his face, waiting for the catch. "'Pure, innocent blood'," he quoted dubiously. "You mean a virgin's?"

"Yes and no."

"What then? I'm guessing you don't mean mine?"

"No. I mean Dean's."

There was a pause.

"Dean's?" Sam repeated, nonplussed.

"Yes."

Sam raised his eyebrows. Cas glanced from his expectant face to Dean's still one. "Obviously I don't mean his blood."

"Obviously," Sam said, his expression unchanged. "Do we know any other Deans ...?"

"None that are mutual, but I did –"

"Cas, get to the point. What do you mean Dean's blood, but not Dean's blood? It's not like we've got any on ice."

"I know. This is where it gets complicated."

"You don't say."

"You know how everything in this universe is strengthened by having a physical force? A corporeal incarnation?"

"Cas, I swear, if you don't spit it out –"

Cas raised a placating hand. "All right, I'm sorry. It's an intricately complicated issue I'm talking about, it's difficult to explain."

"Try."

Cas took a breath and looked down at Dean. "From what we can gather, Dean is battling his demonic self, correct?"

Sam nodded, unsmiling.

"And his body is weakening as a result, correct?"

"Go on," Sam mumbled, his brow starting to furrow.

"Well, then it makes sense to arm him via his body with fresh blood. But not any blood. Because since this fight is Dean versus Dean, I think we'd need to exchange as much of his blood as possible with his own exact mixture, only one that's completely free of not only demonic trace elements but the memories as well."

Sam leant back in his chair, his mouth opening in shocked comprehension. "You're not suggesting what I think you're suggesting, are you?"

"Well, without reading your mind I can't be sure what –"

"Are you suggesting we go back in time and ask Dean for a pint of blood?"

"Actually I'm suggesting I go back in time and ask Dean for blood."

Clearly, Sam was liking this plan less and less. "I thought you said time travel was dangerous and hardly ever done. You know how many times Dean and I've been sent back? Every time either of us 'messes with the flow of time', you or your buddies spend hours lecturing us over it! And, may I remind you, the only good thing that's ever come of it was a box of phoenix ash!"

"I know, Sam. But it's the only thing I can think of to help him. And it's not just a hunch. I believe it will work."

Sam ran a hand through his hair, making it stick up briefly at the back. "Well ... how far back are you thinking of going? It's been almost two years since Dean got the Mark."

"Actually … I was thinking a lot further back than that."

"How far?" Sam looked as though he didn't want to hear the answer.

"1983."

There was a long pause as Sam stared at Cas with an unfathomable expression.

"Why?" he said at last. His voice was low and gruff.

"Because Dean was, and I'm sure you'll agree, at his most innocent before Azazel came into your house in November. That should make his blood all the more potent. Theoretically I could go back to any year before your mother died, but time travel is difficult enough as it is, and my new Grace is untested in the matter. It certainly feels powerful enough to bring me there and back with the blood, but I don't want to risk going back any further."

"Cas." Sam looked as though he hadn't heard a word of what he'd said. "You're talking about going back and asking a kid to give his blood to some strange man who just, what, appears in his room?"

"Yes."

Sam threw his hands in the air in exasperation. "Cas! That's insane! How the hell are you gonna convince him?"

"I'll figure that out when I get there."

"But he'll be a kid! Four years old! How much blood are you gonna take?"

"Just a small vial full. Then I'll be able to duplicate it and make as much as we need."

That seemed to placate Sam – for a moment.

"Cas, this is nuts. What the hell is a four-year-old Dean gonna think of some random dude in a trench coat popping into his room asking for blood?"

It took Cas a second to realise Sam wasn't suggesting he leave the coat behind. "I can erase his memory when I'm done. Or convince him it's a dream. I've thought about this a lot, Sam. It must be done." He hesitated. "But, Sam, I won't be able to change anything. Certain events in time are fixed, and any attempt to alter them could prove catastrophic."

Sam looked up at him, his expression miserable. "Couldn't you just go back and stop him taking the Mark from Cain?"

"No. I thought about that, too, but Cain is well shielded. I'm not even sure how Dean and Crowley found him. I'm sorry. This is our last hope, Sam. I have to go back to 1983."

Sam opened his mouth to argue again but then closed it with a frown. He glanced to Dean. He was silent for a long time, thinking it over.

"All right," he said at last, his voice heavy. "If you think it will save him, do it."